(−)-Δ9-Tetrahydrocannabinol (Δ9-THC), the major psychoactive constituent of marijuana, exerts a broad range of biological effects through its interactions with two cannabinoid (CB) receptor subtypes, CB1 and CB2. CB1 receptors are highly expressed in the central nervous system and to a lesser degree in the periphery in a variety of tissues of the cardiovascular and gastrointestinal systems. By contrast, CB2 receptors are most abundantly expressed in multiple lymphoid organs and cells of the immune system, including spleen, thymus, tonsils, bone marrow, pancreas and mast cells.
The psychotropic effects caused by Δ9-THC and other nonselective CB agonists are mediated by CB1 receptors. These CB1 receptor-mediated effects, such as euphoria, sedation, hypothermia, catalepsy, and anxiety, have limited the development and clinical utility of nonselective CB agonists. Recent studies have demonstrated that CB2 modulators are analgesic in preclinical models of nociceptive and neuropathic pain without causing the adverse side effects associated with CB1 receptor activation. Therefore, compounds that selectively target CB2 receptors are an attractive approach for the development of novel analgesics.
Pain is the most common symptom of disease and the most frequent complaint with which patients present to physicians. Pain is commonly segmented by duration (acute vs. chronic), intensity (mild, moderate, and severe), and type (nociceptive vs. neuropathic).
Nociceptive pain is the most well known type of pain, and is caused by tissue injury detected by nociceptors at the site of injury. After the injury, the site becomes a source of ongoing pain and tenderness. This pain and tenderness are considered “acute” nociceptive pain. This pain and tenderness gradually diminish as healing progresses and disappear when healing is complete. Examples of acute nociceptive pain include surgical procedures (post-op pain) and bone fractures. Even though there may be no permanent nerve damage, “chronic” nociceptive pain results from some conditions when pain extends beyond six months. Examples of chronic nociceptive pain include osteoarthritis, rheumatoid arthritis, and musculoskeletal conditions (e.g., back pain), cancer pain, etc.
Neuropathic pain is defined as “pain initiated or caused by a primary lesion or dysfunction in the nervous system” by the International Association for the Study of Pain. Neuropathic pain is not associated with nociceptive stimulation, although the passage of nerve impulses that is ultimately perceived as pain by the brain is the same in both nociceptive and neuropathic pain. The term neuropathic pain encompasses a wide range of pain syndromes of diverse etiologies. The three most commonly diagnosed pain types of neuropathic nature are diabetic neuropathy, cancer neuropathy, and HIV pain. In addition, neuropathic pain is diagnosed in patients with a wide range of other disorders, including trigeminal neuralgia, post-herpetic neuralgia, traumatic neuralgia, phantom limb, as well as a number of other disorders of ill-defined or unknown origin.
Managing the spectrum of pain etiologies remains a major public health problem and both patients and clinicians are seeking improved strategies to effectively manage pain. No currently available therapies or drugs effectively treat all types of nociceptive and neuropathic pain states. Compounds described herein are novel CB2 receptor modulators that have utility in treating pain, including nociceptive and neuropathic pain.